During a U.N. Security Council debate Wednesday, Iranian Ambassador Mohammad Khazaee said "we have never and will not engage in such a despicable attempt on…innocent people," Reuters reported.

"Such a terrorist operation could only be planned and carried out by the same regime whose short history is full of state terrorism operations and assassinations aimed [at] implicating others for narrow political gains," the Islamic Republic's ambassador continued.

Khazaee said he could provide "many examples showing that this regime killed its own citizens and innocent Jewish people during the last couple of decades."

Meanwhile, in a joint press conference with U.S. counterterrorism chief John Brennan, Bulgarian Prime Minister Boyko Borisov told reporters the terror cell that carried out last week's attack used "leased vehicles."

"They moved in different cities so as not to be seen together," Borisov said, "and no two of them can be seen in one place on any security camera."

Borisov said the cell, which he called "exceptionally skilled," went to great lengths to observe "absolute secrecy."

"There was absolutely no chance of preventing such an act of violence," he continued. "We could have only detected it by chance or if we had been informed by the services that such activities were underway in Bulgaria."

Borisov said investigators have yet to learn the bomber's name, but they have pinpointed the flight he arrived on.

"We do not know his identity, but it is known when he has arrived, the presumed flight and where he came from," Borisov told reporters.

Surveillance videos captured the killer on camera so authorities know he was wearing a wig and clothing that disguised his identity.

Israeli officials believe the Lebanese-based Hezbollah terror organization and its Iranian sponsor are behind the attack.

Prime Minister Benjamin Netanyahu said the government had "concrete evidence" linking Hezbollah to the attack.

Israel Defense Forces Chief of Staff Lt. Gen. Benny Gantz said the IDF would "find a way to respond to this attack."

"We will know how to do it judiciously," Gantz said. "Ultimately, the response will come."